1. Field of the Invention
The present invention relates to a radiation source container that accommodates a radiation source capsule in which a radiation source is sealed, and to a method of extending the sealing life of a radiation source capsule accommodated in the radiation source container.
2. Description of the Related Art
Measurement equipment employing radiation to measure for example the thickness, level, density or water content of a measurement subject by measurements utilizing radiation is known. Also, regarding the various types of such devices utilizing radiation, safety standards in relation to sealing of the radiation source are laid down in Japanese Standard JIS Z4821 and safety standards regarding the radiation source container that accommodates the sealed radiation source are laid down in Japanese Standard JIS Z4614, respectively. Examples are to be found in: Technical Explanation of Measurement Equipment Utilizing Radiation [online], 2007, Japan Electric Measuring Instruments Manufacturers' Association Inc [retrieved 29 Jan. 2009] Internet <URL: http://www.jemima.or.jp/> (hereinbelow referred to as non-patent reference 1).
A conventional radiation source container using γ rays as the radiation source in accordance with these standards will now be described with reference to FIG. 1A and FIG. 1B. FIG. 1A is a plan view of a conventional radiation source container 100 in a condition with the container lid 7b removed from the direction of the top; FIG. 1B is a cross-sectional view seen from the direction of the broken line arrows Xa-Xb of FIG. 1A.
In FIG. 1A and FIG. 1B, the radiation source container 100 comprises a radiation source capsule 1 that emits γ rays and that is hermetically sealed with metal such as stainless steel, a radiation source holder 4 that accommodates this radiation source capsule 1, having an aperture B that emits γ rays emitted from an radiation port A in the middle of the top of the radiation source capsule 1 unidirectionally and whereby screening is effected such that γ rays cannot leak except from this aperture B, and on which holder this radiation source capsule 1 is fixed in such a manner that it is difficult to attach or detach, and a radiation source holder accommodating container 7 that accommodates the radiation source holder 4.
This radiation source holder accommodating container 7 comprises a radiation source holder accommodating container lid 7b having an irradiation window C provided with a window plate 7b1 formed of a metal plate, such as aluminum, that is transparent to γ rays, and a radiation source holder accommodating container main unit 7a of this radiation source holder accommodating container lid 7b: γ rays are emitted from this irradiation window C.
The radiation source holder accommodating container 7 further comprises, in its interior, an electrically operated shutter 6 that is operated by a control signal from outside, and a manually operated shutter 5 that is operated by hand so as to screen emitted γ rays from the irradiation window C provided between the radiation port A of the radiation source capsule 1 and the irradiation window C.
In more detail, the radiation source holder 4 comprises a capsule fixing member 2a comprising a lid 2a1 and a seat 2a2 whereby the radiation source capsule 1 is fixed in the interior of this radiation source holder 4, a holder side plate 4a with a lead plate stuck thereon so as to prevent leakage of γ rays from outside the aperture B, and a holder base plate 4b. 
Also, when the radiation source container 100 is stored or transported, the manually operated shutter 5 is manually operated to operate a screening plate 5a made of a lead plate or tungsten plate, to effect screening so that there is no leakage of γ rays emitted from the aperture B.
When the radiation source container 10 is incorporated in a measuring instrument employing radiation, the electrically operated shutter 6 controls whether or not other control devices constituting the measuring instrument employing radiation are subjected to irradiation or screened therefrom; in this electrically operated shutter 6, a screening plate 6d made of a lead plate or tungsten plate is mounted on a mounting base 6c; the mounting base 6c is mounted on a rotary shaft of a rotary solenoid 6b and is rotated by this rotary solenoid (coil) 6b; emission of γ rays from the aperture B is controlled by opening/closing the screening plate 6d; irradiation of outside the radiation source holder accommodating container 7 by γ rays from the irradiation window C is thereby controlled.
The radiation source capsule 1 accommodated in the radiation source container 100 constructed in this way is usually sealed by welding in a stainless steel capsule; however, the manufacturers of the radiation source capsule 1 recommend that the period of use of this welded seal should be no more than about 15 years; if the radiation source container is to be used for more than this, replacement of the radiation source capsule 1 is recommended.
Also, techniques have been disclosed for improving the performance of a level meter employing such γ rays. An example is to be found in Japanese Patent Number 3063488 (hereinbelow referred to as patent reference 1).
However, in the case of 241Am (radiation source), the half-life is 432 years and even in the case of 137Cs (radiation source) the half-life is 30 years. Thus, from the point of view of the half-life of the radiation source, it would be possible to employ the radiation source container for longer than the recommended period of use of the welded seal. Thus, replacing the radiation source capsule within the recommended period of use of the welded seal presents the problems not only that the task of replacing each one of devices that are employed in large quantity is considerable, but also that of wastefully discarding radiation capsules.
Consideration has therefore been given to employing the radiation sources for longer by improving the recommended period of use of the seal by re-doing the welded seal of the entire radiation capsule. However, such recycled capsules are subject to the problem that the amount of radiation is reduced so that the initial amount of radiation cannot be obtained, and also the problem that the guarantee of sealing is unreliable, so that a product of low reliability is obtained.